Screw stripping torque

Stripping torque n. (1) Of a self-tapping screw, the twisting moment in Nm required to strip the threads formed by the screw in a softer material. (2) Of a molded-in insert, the torque required to break the mechanical bond between the plastic and the insert s knurled surface. 

TABLE S-6. DRIVING AND STRIPPING TORQUES AND PULL-OUT STRENGTHS OF SELF-TAPPING SCREWS IN CELANEX 3300 ... 

Table 5-6 presents the optimum pilot hole diameter, and driving and stripping torques of self-tapping screws for 30% glass-reinforced polyester. Both thread forming screws and thread cutting screws can be used. 

Driving torque values indicate the torque necessary to drive the screws into the pilot holes. Stripping torque values indicate the tightening torques which will strip the threads formed by the self-tapping screws. In many applications, a low driving torque with a substantial drive/strip ratio is of primary importance. 

Stripping Torque n (1) Of a self-tapping screw, the twisting moment in newton meter required to strip... 

Most people are familiar with overdriving a screw into a plastic boss and stripping the screw. The torque required to drive a screw increases (shown as a linear increase in Fignre 12.16) as the thread engagement increases up until the screw head is seated at Ts. If the additional torqne is applied, there will be a small amount of elastic torsional deflection until the screw starts to strip the plastic. 

In another case where the twin-screw extruder was used, the rubber and plastic were melt mixed with all ingredients in a similar manner as described in blend compositions for static vulcanizations. The product was then dumped, cooled, and granulated. The premixed granules were then fed into a twin-screw extruder where a very narrow temperature profile was maintained with a relative high compression (2 1), and the screw speed was adjusted depending on the final torque and the flow behavior of the extruded stock. The stock was cured by shear force and temperature enforced by the twin-screw extruder. The dynamically crosslinked blend was taken out in the form of a strip or solid rod to determine the... 

The design of the threads requires control, to prevent excessive shear, resulting in stripping the threads when torqued, and also to limit hoop stresses that can result in tensile failure. Although the mechanics of stress analysis for screw threads are readily available, the equations for them can be rather complicated. 

Although a boss cap does not increase the load bearing capacity of the boss, it does promote positive alignment of the screw and increases the initial driving torque required to insert the screw. This reduces the possibility of stripping boss threads during insertion. 

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